Drosophila and the moth, Manduca sexta, are used as models to determine how steroid hormones control proliferation and differentiation during neuronal development. One objective is to determine the role of the nitric oxide (NO), cyclic GMP, and cGMP-dependent protein kinase (PKG) pathway in synapse formation. The time during neuronal development when the various synthetic enzymes are present will be determined. By the use of inhibitors or mutants, the steps in this pathway will be blocked and the effect on the subsequent development of the neuromuscular junction (Manduca) or the visual centers (Drosophila) will be assessed by light microscopy. Double mutant or inhibitor-mutant combinations will help establish functional interactions. Based on the anatomical results, the physiological effects of interfering with components of the pathway will be assessed. The endocrine regulation of components of the pathway will also be examined. An important issue is the signal causing neurons to become NO sensitive. Genetic and surgical methods will be used to remove normal targets and the effects on the onset of NO sensitivity assessed. The second objective is to understand how quantitative changes in steroid levels can shift neuroblasts from a proliferative to a differentiative response. Neurogenesis in the optic lobes of Manduca requires low levels of ecdysone (E) or 20 OH-ecdysone (20E) but is shifted into an all-or- none differentiative response by high levels of 2oE. A transcription factor (MHR13) that may be involved in the proliferative actions of E has been identified. Studies involve cloning MHR13 cDNA, characterizing its expression and endocrine regulation, and isolating the Drosophila homologue. The impact of altering proliferation patterns on the differentiation of the optic lobes will be determined.